A well-known and typical feature of the common AR-pattern firearm is a receiver extension tube that houses a recoil buffer and spring, providing an axially aligned enclosure for reward movement of a standard bolt carrier assembly. Although acting as an extension of the upper receiver, it is mounted to the lower receiver and requires that the recoil buffer and spring be separable from the bolt carrier assembly. When configured as a rifle, this extension is accommodated within and supports a buttstock. However, this structure makes it challenging to use a folding buttstock and, if damaged, may render the firearm inoperable. Additionally, this extension can be an awkward protrusion when configured as a handgun. Some designs have shortened, but not eliminated, the extension tube.
The standard T-shaped rear charging handle is not ergonomically ideal in location and, in direct impingement systems, can allow hot gas to escape the upper receiver near the user's face. Other side charging handle designs for the AR-pattern firearm have required extensive modification and used many nonstandard parts.